Some type of vacuum cleaners include a generally upright tank and a lid which is removably connected to and which generally closes off the upper end of the tank. A vacuum source connected to the lid draws process air, i.e., air carrying dust and debris, through an inlet opening in the tank and then through an exit opening in the lid. A generally cylindrical foam or paper filter with an open upper end in communication with the exit opening in the lid and a closed lower end is typically connected to the underside of the lid and extends downwardly into the canister.
During normal operation of the vacuum cleaner, the vacuum source draws process air into the tank and then through the pleated filter for removal of particulate from the air stream, whereupon the air exits the upper end of the filter and through the exit opening in the lid. With this arrangement, the vacuum source causes a build-up of particulate on the side of the filter. After a certain quantity of contaminant or debris builds up on the outside of the filter, or when the vacuum source is turned off, the build-up may or may not fall from the filter.
A typical prior art solution has been to cover the filter with a pre-filter bag. This bag is made of tightly woven fibers in order to stop dust from entering the filter. When the vacuum is turned on, the air flow through the filter pulls the bag into contact with the filter. When the vacuum is turned off, the bag returns to its original shape, allowing dirt to fall off the bag. While this solution has been used with vacuums that collect dry materials, it is problematic to use with vacuums used for collecting wet materials because the suction would pull water into the bag. Because of the bag construction, the water drains from the bag very slowly.